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The precise semantic distinction that some people like to pretend exists between "law" and "theory" was even less in evidence in the misty days of yore, than it is now. In short, you are trying to force dead people to adopt your terminology and they won't. AKA your fourth grade teacher lied to you.
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dmckee♦Sep 26 '11 at 3:01

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I'm temped to close this as off-topic (or migrate to English.SE?). Thoughts from the crowd?
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dmckee♦Sep 26 '11 at 3:04

There was a time when I would definitely agree; now I only probably agree. One could argue (though I'm not saying one would necessarily be correct) that this is a question about scientific terminology, and is perhaps specific enough not to be appropriate on English. I'd at least ask in their chat or meta to see if it's appropriate there. If not... well, it's not a great question, but I'd be inclined to leave it around if nothing equivalent has already been asked. People are often confused about the meanings of "theory" and "law" in science and it'd be nice to have an explanation to point to.
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David Z♦Sep 26 '11 at 3:23

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In addition to what @david said, the people at english.se would likely reinforce this erroneous idea. We would do a disservice to science in general by taking to answer this.
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Colin KSep 26 '11 at 5:21

4 Answers
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There are theories which are just not going to be disproved, ever, ever, ever. The story people tell about this is a "noble lie", in the sense that it is designed to get them to distrust authority, and this is good. But the way this is done by drawing a false equivalency between garbage that gets accepted and disseminated, like Aristotle, and science that get superseded in certain domains, like Newton. Aristotle's stuff was completely wrong, and Newton's stuff is completely right. Newton made a few mistakes, but by and large, was honest and accurate scientist. What he discovered does not get undiscovered, it will stay valid forever. It can only be forgotten.

The statement that Newton's theory is valid for macroscopic objects of normal atomic density and size less than the sun, moving at less than $10^{-4}$ of the speed of light, to a certain accuracy which is fairly perfect, will never be overthrown. It is just a fact. These types of facts when organized with some mathematics are usually called the "laws", and Newton's laws continue to be laws of nature in certain restricted domains even after Newton's theory is superseded. Newton's theory was much more overarching than the modest claim above: Newton claimed that his laws are true for all regimes--- it aspires to be a complete theory. In this aspiration, it fails, and it gets modified at high velocities, small distances, and high densities.

But you never backpeddle on the experimentally established facts. You never unlearn Kepler's laws, you never unlearn that you can make as much heat as you want by drilling a cannon, you never unlearn that Aristotle is garbage, and you will never unlearn that quarks exist. These are laws of nature, forever immutable, like the restricted Newton's laws.

Similarly, relativity will never be superseded in its domains of validity. If you discover that relativity is wrong, it will be in another regime which hasn't been seen. The way science proceeds is different from the stories. It's a ratchet, with forward motion, but its wobbly, and it takes a long time for the tooth to drop. The design is to ensure that bullshit like Aristotle is not made dogma.

The design of science was for another time, with literature which was not instantaneous, and no ability for people to cross check everything by themselves. We live in a better media environment, and this will mean that the mechanism of science can relax a little. It is much harder for bullshit like Aristotle to get accepted today, because anyone can call it out publically with no effort.

A theory is not able to be a law, since it's just an explanation that can always be disproven. So how is there a law of a theory?

You're probably thinking of the popular (non-science) definitions of the words "theory" and "law." But scientists mean something entirely different when they talk about theories and laws. Of course, given human nature, it's not always entirely clear what we do mean by those words; the definition you'll get varies depending on who you ask and in what context the words are used ;-)

Based on my experience, though, this definition describes pretty well what scientists usually mean when they say "theory":

theory: a coherent group of tested general propositions, commonly regarded as correct, that can be used as principles of explanation and prediction for a class of phenomena

Of course, you have to know that in science, "correct" doesn't mean that it works for every case ever, just that it works often enough to be useful.

I would add to that this definition (which I just made up) for "law":

law: a rule or principle, usually based on observations or calculations, that allows one to predict the behavior of a physical system in some manner

In other words, a "law" is one of the tested general propositions that makes up a "theory."

I would say that the word "Law", in the context of science, is a bit antiquated. It was originally invoked because it was thought that these were actual laws ordained by some sort of overseeing creator. If you so choose, you could continue to see them that way, although that's not really an idea germane to doing science.

The word "Law" itself predates our modern, nuanced, falsification-based way of thinking about scientific theories: it came from an era when we thought we could discover the truth by means of pure reason, and get a simple answer on the first try if we were only careful enough. The word "law" captures that easy, authoritative confidence. But the early 20th century taught us that things don't quite work that way.

In mathematics, there are also "laws": the distributive law that a(b+c) = ab + ac. But this is not something which we declare to be true, most of the time; this is something we discovered about the arithmetic that we use — it's a theorem, and one which happens to be useful sometimes for doing calculations. It's a rule for computation, and it's in this sense today that you should probably understand the "laws" of a theory: they are rules for you, to guide you in understanding what's going on.

Law - is only based in observation that a system under the same causes will get the same outcome. The laws are constant. Better measures can make better statements. We can have, and we have phenomological laws without theory; A Law states: 'it is so'.

Theory - They are based in a set of postulates, generally accepted apriori as correct , as the basis to the derivation of an explanation of the observed nature behaviour in a specific domain. A theory is always a 'may be the case'. It is a mental construct to explain the laws. The first theories were based in 'god's personalities. The god of fire, etc...

By example - a law like this one in Aristoteles time: a block of water ice will be transformed into liquid if heated above 0º (let's think that they have sensible finger as a thermometer). A fellow took himself to the top of a high mountain and verified that the ice became liquid at a lower than 0º temp. He will restate the terms of the law and constructed a theory: the god of the fire don't love the god of the ice; idem to the god that rules in the high mountains. Now we have elaborated explanations (theories) based in the energy concept. In the past the will and power of the gods are the 'energy'. With respect to the same 'melting ice' law, we beleived, thru time, in different explanatory theories.

The distinction between 'law' and 'theory' is central to physics.
With the same bolding : 'The domain of physics is the observable world'. If we can not devise a way to put it under test then 'it is not physics'. Until then it is only: 'a maybe physics - we don't know yet.' And this strongs statements are true 'by definition'. The practice has been tolerant and permissive. IMO it is important emphasise the correct vision.

When a theory must fall - Based in the postulates a new theory must, if capable, and to honour the past, make derivations to predict an effect that could stay unnoticed or unobserved until now, and possibly enlarge/merge two or more distinct theories that must evade under the new larger umbrella. A well behaved theory, to honour the past must fall when an anomaly was found. New conflicting data, and data only, can and must terminate the 'old theory' or, if it is the case, that a new theory has less postulates than the previous one (Occan razor criterium).

Laws of theorys - yes. If the subject of the study is the evolution of theories we can possibly make some observations that we verify that ever hold, without exception. That are 'laws'.

Simply stated, while a law notes that something happens, a theory
explains why and how something happens.
However, fundamental changes to the laws are extremely unlikely, since
this would imply a change to experimental facts they were derived from
in the first place. Well-established laws have indeed been invalidated
in some special cases, but the new formulations created to explain the
discrepancies can be said to generalize upon, rather than overthrow,
the originals.